Electrostatographic printing is well known and is commonly used in making photocopies of an original document. See, generally, R. M. Schaffert, "Electrophotography," The Focal Press, New York, 1965.
Electrostatographic printing includes the well-known process of transfer. In transfer, charged toner particles from an image-bearing photoreceptor member are transferred to an image support substrate, such as a copy sheet. Transfer is accomplished at a transfer station, wherein the transfer occurs by electrostatically overcoming adhesive forces holding the toner particles to the image-bearing member, thus transferring the developed toner image to the substrate.
In conventional electrostatographic machines, transfer is achieved by transporting the image support substrate into the area of the transfer station. The transfer station applies electrostatic force fields sufficient to overcome the adhesive forces holding the toner to the photoreceptor surface in order to attract and transfer the toner particles onto the image support substrate. In general, such electrostatic force fields are generated by means of electrostatic induction using a corona-generating device such as, for example, a dicorotron. The copy sheet is placed in direct contact with the developed toner image on the photoreceptor surface while the reverse side of the copy sheet is exposed to a corona discharge. This corona discharge generates ions having a polarity opposite to that of the toner particles, thereby electrostatically attracting and transferring the toner particles from the photoreceptive member to the image support substrate.
During electrostatic transfer of a toner image to a copy sheet, it is important for the copy sheet to be held in direct, uniform and intimate contact with the photoconductive surface and the toner image developed thereon. Unfortunately, however, the interface between the photoreceptive surface and the copy substrate is not always optimal. Various substrate conditions such as copy sheets being mishandled, wrinkled, creased, left exposed to the environment, or previously processed by a heat and pressure fusing or fixing operation, result in insufficient substrate contact with the photoreceptor surface during transfer. This substrate condition creates spaces or air gaps between the developed image on the photoreceptor surface and the copy sheet. The air gaps, in turn, impair transfer of the toner image, thus causing copy defects.
It is known to use transfer-assist pressure blades in the transfer process. Such transfer-assist pressure blades mechanically press the copy substrate into substantially uniform intimate contact with the image-bearing surface, just prior to the build-up of the transfer electrostatic field. Moreover, by flattening the substrate against the photoreceptor, the transfer-assist pressure blade thus eliminates the foregoing air break-down caused by substrate cockle. A further purpose of the transfer-assist pressure blade is to reshape and optimize the transfer field profile under the transfer charging device. Some examples of transfer-assist pressure blades may be found in U.S. Pat. No. 5,923,921 to William M. OuYang, et al., and references cited therein. See also, U.S. Pat. No. 5,568,238 to William G. Osbourne, et al., and references cited therein. One example of a current transfer-assist pressure blade consists of a single pressure blade made of a dielectric material.
As noted in the foregoing William G. Osbourne et al. patent, it is known that electrostatic interaction may occur between the transfer-assist pressure blade member and the copy substrate. This is because the transfer-assist pressure blade is located in the transfer zone between the transfer corona-generating device such as, for example a dicorotron, and the photoreceptor. As a result, a measurable electrostatic charge is imparted on the blade member by the transfer dicorotron. Once the transfer-assist pressure blade is charged, it repels any additional charges away from itself, thus leaving a small area that is devoid of charge on the copy substrate adjacent to the transfer-assist pressure blade tip. As explained below, this unwanted electrostatic charge on the pressure blade causes a problem in multiple-toner color printing known as "toner drag out".
As is known, color printing may be achieved by using multiple layers of different colored toners. In a typical color process, for example, three colored toners are used, comprising magenta, yellow and cyan. In a typical three-layer image, the top cyan layer is relatively low-charged compared to the layers below it, or to the charge of one-layer and two-layer images. The low charge on the cyan image, together with its relatively far distance from the photoreceptor, cause the cyan image to be susceptible to forces which may pull the cyan away from the image charge on the photoreceptor.
When the image/substrate/photoreceptor "sandwich" passes under the charged transfer-assist pressure blade, the "fluffy" cyan layer is attracted both to the unwanted charge on the transfer-assist pressure blade and to the image charge on the photoreceptor. Next, the image passes under the substrate charge void area adjacent to the transfer-assist pressure blade. Here the attraction force of the cyan layer to the charged transfer-assist pressure blade overcomes the electrostatic photoreceptor image force. Because the transfer dicorotron has not sufficiently charged the substrate to overcome the electrostatic forces of either the charged image or the charged transfer-assist pressure blade, a tangential transfer-assist pressure blade electrostatic force causes the image to be "dragged out" in the upstream process direction. Here "tangential" indicates the direction parallel to the process direction.
It will be understood that this toner drag-out problem is primarily related to the top toner layer of the multi-layer toner image. Thus, when the top toner layer is cyan, this problem is known as transfer "cyan drag out". Moreover, in another hypothetical multi-layer toner image with magenta as the top layer, this problem is known as transfer "magenta drag out".
As a result, to solve the problem of "toner drag out", there is a need for an improved transfer apparatus that substantially eliminates the unwanted electrostatic charge on the transfer-assist pressure blade.